Process for sulfide-coating ferrous alloys



United States Patent PROCESS FOR SULFIDE-COATING FERROUS ALLOYS Emerson H. Newton, Arlington, and John L. Sienczyk, Chelsea, Mass., assignors to Arthur D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts No Drawing. Application March 12, 1957 Serial No. 645,418

Claims. (Cl. 148-614) This invention relates to the rapid formation of sulfide coatings on ferrous alloys. These coatings prevent galling and seizing of the ferrous alloys during mechanical forming, protect such alloys from corrosion in many environments and may act as a base for subsequent organic coatmgs.

When ferrous alloys are to be rolled or formed it is very common to stretch them beyond their elastic limit. In such forming processes, particularly in the process of tube drawing, large pressures are exerted upon the surfaces of the ferrous alloys to be treated so that high temperatures exist at the interface between the metal and the mandrel. These conditions require that some type of lubrication be furnished during the process either in the form of a specialized coating on the ferrous alloy to be treated and/or in the form of lubricating oils or greases. In the absence of such lubrication, excessive galling occurs in drawing tubing material over a mandrel, and material placed in a die or stamping machine cannot be freed from the die or stamp. These are cited as examples to show the need for such lubrication.

Many ways of providing the proper protection of lubrication are known in the art such as applying a. thin copper plating, and using liquid lubricants along with coatings such as sulfides, oxides and molybdenum sulfide. Heretofore, the known processes for producing sulfide and oxide coatings on ferrous alloys have involved either high-temperature gas treatments, molten salt baths, or high boiling point, highly concentrated aqueous solutions. Each of these processes is a relatively slow one, and is difficult to adapt to a continuous process. The use of sulfide coatings on steels as a lubricating film during mechanical forming is well known. It has been postulated that sulfide coatings tend to prevent galling of the metal being formed to the die or mandrel because the sulfide compounds are present on the surface as crystals which are capable of shearing preferentially in one plane so that metal-to-metal contact never occurs. Such sulfide coatings are, of course, protection against a limited amount of wear and are normally applied where the protection is desired in forming or shaping processes. Hitherto, sulfide coatings have been deposited with some difficulty in that a great amount of time has been required.

It would therefore be desirable to have a good sulfide coating which could be used with or without an additional lubricant and which could be applied very rapidly to the surface of ferrous alloys.

Furthermore, it would be desirable to provide a sulfide-coating deposition process which would be continuous, would use an aqueous solution, and would not require the use of excessively high temperatures.

It is therefore an object of this invention to provide an effective coating on ferrous alloys which will permit their forming and shaping without galling or seizing under conditions wherein the metal is severely worked and plastically deformed at a rapid rate. Another object is-to provide an effective sulfide coating on ferrous alloys 2,829,995 Patented Apr. s, 1958 The process of this invention may be accomplished by the following general steps.

(1) Cleaning the steel to be coated in an activating bath which removes small amounts of metal from the surface and leaves a fresh, highly active surface exposed;

(2) Rinsing the article in a weak acid rinsing bath;

(3) Removing excess rinse water;

(4) Immersing of the rinsed article in a sulfiding bath; and

(5) Finally rinsing and drying the coated article.

The steps outlined above will be discussed in detail.

The cleaning and activating steps serve to etch the surface of the ferrous alloy and to activate it, apparently by forming on the surface a controlled amount of ferrous ions. This is done by immersing the article to be coated in an aqueous solution containing originally ferric ions supplied by a ferric salt of a strong mineral acid, such as ferric salts of sulfuric acid, hydrochloric acid or nitric acid. Since the activating solution should have a pH range from about 0.3 to about 2.0, the strong mineral acid should be capable of giving a pH of at least 2 in an aqueous solution. It has been noted that after ferrous alloys have been immersed in this activating solution, the solution contains ferrous ions as well as ferric ions. Ferrous ions are formed partly by the attack on the steel surface of the acid solution which releases hydrogen gas:

In order to keep the activating solution in proper condition, it is necessary to balance the amount of ferric and ferrous ions to maintain the proper pH. This balance may be achieved by adding more of the ferric salt or by adding acid. However, we have found that it is necessary to maintain sufiicient ferric ions in the activating bath. Since the pH is a function of the amount of these ferric ions, the control of the pH by the addition of ferric salts is a good measure of ferric ion concentration.

Since it appears that it is necesary in the final formation of the sulfide coating to have ferrous ions present on the surface immediately prior to immersion in the sulfiding bath, it may be postulated that these ferrous ions result from the following type of oxidation-reduction reactions:

This then would explain why tiis preferable to control the pH of the activating solution by addition of ferric salt rather than of acid alone, since the ferric ions appear to be an important source for the finally required ferrous 10I1S.

In the case of ferric sulfate, we have found it preferable to have the following range of concentrations in an aqueous solution:

Grams/liter Anhydrous ferric sulfate 50-500 Ferric ion analyzed 2-140 Ferrous ion analyzed 1- 78 As pointed out above the balance between the ferric ion and the ferrous ion may be maintained by, proper pH control.

The temperature at which the activating bath should be maintained should range from about 160 Ffto the boiling point of the activating bath. We have found that temperatures less than about 160 F. are not desirable because the action of the bath is too slow.

The time of immersion of the ferrous alloy article to be' Excess activating bath may beremoved by any suitable process such as by draining, passing through rollers, etc.

We have found that the use of the rinsing bath is a critical point in this invention and that-the control of the pH in the rinsing bath is most important. ThepH of this rinsing bath should be between about 2 and 3. Thus, if the rinsing bath is too acid the final coating to be formed will not adhere to the ferrous alloy surface but will form a porous coating which flakes off easily. On the other hand, if the rinsing bath is not sufficiently acid-the coating will not form at all. This is probably because the ferrous ion tends to be oxidized or to be precipitated as hydroxides. It is also essential to control the thickness of the layer or film of the rinsing bath retained on the surface after once the article to be coated'has been removed from this bath. We have found it convenient either to squeeze off excess rinsing bath by passing the article through a controlling roller if it is flat, or by draining it momentarily if it is a complex shape. Simple experimental trials may be made to determine the optimum time for draining or the exact amount of rolling or wiping which must be accomplished for each piece or shape of steel to be protected as it is removed from the activating bath and from the rinsing bath.

The rinsing bath may be kept at temperatures ranging from room temperature to about 110 F. and immersion time in the rinsing bath may vary from about 1 to seconds.

It would appear from experimental data and from the formation of the final coating on the ferrous alloys that this rinsing bath is a means for controlling the exact amount of ferrous ions which are required on the metal surface for the next step of sulfiding the surface. Thus, the control of the pH in the rinsing bath is, in fact, a means of controlling the ferrous ion concentration on the surface of the metal.

The sulfiding bath comprises an aqueous solution of polysulfides and sulfides of an alkali or an alkaline earth metal. The sulfiding bath is conveniently prepared by mixing an inorganic alkali or alkaline earth metal compound and flowers of sulfur in water and boiling until the resulting polysulfides, along with any sulfides, are in solution. The pH of this sulfiding bath should be on the alkaline side, preferably above about 7.5.

It is desirable to have a high concentration of the soluble polysulfide and sulfide ions present in the sulfiding bath. Presumably, the ferrous ions react with the soluble polysulfide and sulfide ions to form a ferrous sulfide coating by a simple metathetical reaction:

which are suitable for inclusionin the sulfiding bath in-v clude but are not limited to sodium hydroxide, calcium sulfide, and calcium oxide. Typical satisfactory sulfiding baths contain, for example, 150-200 grams of sodium hydroxide and 150-200 grams of flowers of sulfur in one liter of water; or 10-20 grams of calcium sulfide and 10-30 grams of flowers of sulfur in one liter of water; or 10-20 grams of calcium oxide and 1030 grams of flowers of sulfur in one liter of water.

The temperature at which the sulfiding takes place should range from about room temperature to about F. If the temperature rises much above 130 F. there is the possibility of degradation of the coating.

After the article to be coated has been dipped in the sulfiding bath and the ferrous sulfide has formed on the surface, it is removed, rinsed in hot water and the coating allowed to dry. Drying may be accomplished by exposure to air, but may be hastened by such well-known expedients as infrared radiation, placing in an oven, or passing hot air over the surface.

Once the coating has been dried, it will adhere tightly to the metal surface.

The following examples are given as illustrative of this invention and are not meant to limit it in any way.

Example I A 3,000-foot long reel of cold rolled 1010 carbon steel strip about 1.4 inches wide and 0.014 inch thick was precleaned in a proprietary alkaline steel cleaner and coated with a black adherent sulfide film by passing the strip through the following baths at a speed of 200 feet per minute and under the conditions noted below. An analysis of the activating bath showed it to contain the equivalent of 45 grams of ferric ion introduced as ferric sulfate and the equivalent of 32 grams of ferrous ion per liter of water. A pH of 0.57 was maintained and the temperature of the bath was 200 F. The immersion time was 1.5 seconds. After being drawn from this activating bath the steel strip was momentarily drained and was passed through an aqueous solution of sulfuric acid as a rinse which had a pH of 2.95 and which was maintained at room temperature. The strip was immersed'in this rinse for 1.5 seconds. Excess rinse liquid was removed continually by means of a roller and the strip then passed into the sulfiding bath consisting of 10 grams of calcium sulfide, 20 grams of flowers of sulfur per liter of water. This bath had been boiled to achieve solution and formation of the polysulfides of calcium (CaS and then cooled. It had a pH of 9.5. The temperature of the sulfiding bath was maintained at 130 F. and immersion was for 1 second. The strip was rinsed in hot'water, dried, and rereeled. A black adherent sulfide coating was obtained having a weight of coating equivalent to 63 milligrams per square foot.

Example II A 3,000 footreel of cold rolled 1010 carbon steel was passed at the rate of 200 feet per minute through the following baths. The activating bath consisted of an aqueous solution analyzed as containing the equivalent of 14 grams of ferric ion and the equivalent of 70 grams of ferrous ion per liter. A pH of 1.1 was maintained and the temperature of the bath was between 196 and 211 F. After removal from the activating bath and rolling off the excess liquid, the strip was rinsed in a sulfuric acid rinse having a pH of 2.25 and maintained at a temperature of 108 F. After rinsing and rolling oflf excess fluid, the strip was then passed through a sulfiding bath having a pH of 9.5 and being comprised of 10 grams of calcium sulfide and 20 grams per liter of flowers of sulfur per liter prepared in the manner described in Example I. The temperature of the sulfiding solution was maintained at 91 F. Since the treating tanks were about 5 feet long, the immersion time for this strip was about 1.5 seconds. The coated strip was washed and then dried by infrared radiation. The reel had a dense black adherent coating weighing about 107 milligrams per squarefoot.

A small piece of low-carbon steel can-stockwas given a sulfide coating by passing it through the following baths in the order indicated. After a preliminary cleaning the test piece was immersed for 5 seconds in an activating bath, removed, and excess liquid drained off. The activating bath consisted of 300 grams of FeCl .6H O dissolved in one liter of water. The pH of this bath was 1.0 and it was maintained at 190 F. The rinsing bath 2. 500 grams Fe (SO .7H 0 in one liter of water,

pH of 1.1 3. 500 grams Fe(NO pH of 0.4 Rinsing baths:

1. Aqueous solution of HCl, pH of 2.5 2. Aqueous solution of H 80 pH of 2.3 3. Aqueous solution of HNO pH of 2.4 Sulfiding baths:

.9H O in one liter of water,

Was an aqueous HCl solution made up to have a pH of grafns NaOH 150 grams flowers of sulfur 2.5. After momentary draining, the test sample was In onehter of Water PH=123 dipped in the sulfiding bath at room temperature. This granjls cast 30 grams flowers of sulfur, sulfiding bath had been prepared by heating to boiling 111 one 11161 of Water PH=11.1 for minutes a mixture of 10 grams of calcium sulfide, I5 3. grams CaO, grams flowers of sulfur, 30 grams of flowers of sulfur and one liter of water. The in one liter of water pH=12.4

Activating bath Rinsing bath Sulfiding bath Type of ferrous alloy Results No. F. See. No. F. See. No. F. Sec.

1035 carbon steel 2 180 2 2 l 73 2 3 96 2 Heavy, black, adherent coatin Do 3 190 2 3 .80 2'2 .95 2 in. High carbon steel (file stock) 2 172 8 2 73 3 1 94 2 Tliinin, adherent black coat- Malleable cast iron 1 200 5 1 73 5 3 92 2 1 50.

Do a 190 2 3 80 2 1 95 2 D0. Pure electrolytic iron 2 215 1 2 73 1 2 95 1 Heavy, black adherent coat- 111 4130 alloy steel 2 195 5 2 73 6 1 95 2 Adhirent, black coating. 400 type stainless 1 210 5 1 73 5 2 106 5 Thlnheldherent gray to black 008 Austenitic type steel 2 180 2 2 73 2 3 96 2 No app reeiable coating formed.

1 Refers to numbered compositions above.

resulting solution containing primarily CaS was per- The ferrous alloys on which a sulfide coating may be mitted to cool to roomtemperature before using. continuously and rapidly deposited in accordance with The test sample was removed from the sulfiding bath, this invention are those in which essentially all of the washed in tap water and then dried. It was evenly coated iron is present in the alpha form. In alloys where the with a thin, black adherent coating. iron is present in the gamma form, such as austenitic l type stainless steels, sulfide coatings cannot be deposited Examp e IV by the process of this invention. A small piece of low-carbon steel can-stock was given (Poatmgs apphed to the manner a sulfide coating by passing it through the following baths scnbed, a ranged m i @qmvalent from 40 to in the order indicated. After a preliminary cleaning 136 mlulgl'ams Per sfluare foot P P the test piece was immersed for 5 seconds in an activati the W coatmg 'F F f an ing hath removed and excess liquid drained 0th The crease in the concentration of the ferric ion in the acaetivafihg hath consisted of 500 grams f Fe(NO3)3 9H2Q t1vat1ng bath. Thus, at grams per liter of ferric suldissolved in one liter of water. The pH of this bath was P an F a coatmg. welght 9 45 mg/ft-z was 9 04 and it was maintained at F. The rinsing hath tamed, while at 500 grams per liter the average coating was an aqueous HNO solution made up to have a pH 50 Welght was 113 of After momentary draining, the test Sample was By the process of this invention 1t is possible todeposit dipped in a sulfide bath prepared in the same manner as a g sulfide on a ferrous alloy coPtalmng mm described in Example In primar ly 111 the alpha form. The coating 1s capable of Upon removal from the Sulfiding bath the test Sample protect ng the metal surface during such processing steps was found to have a heavy, black coating, much of which as mumg and f R f l Where the metal is was removed by washing and wiping ofi. However, afte stretched beyond 1ts elastic limit and where high presdrying there was left a final thin black coating which sures f temPfira'wresi normally encountered in the Was very adherent to the Sample surface processing steps otherwise lead to serious gallmg and In the manner described in Examples I-IV a number selzmgof additional test pieces of various types of ferrous sulfide coatlnfiof fins E may be used P alloys were coated to illustrate the application f the or without the add1t1on of a lubricant such as lard Oll process of this invention to a wide variety of ferrous Q The pal'uclllal' lubnc'fmt to be f alloys determined by the severity of plastic deformation to which Three difierent activating baths, with rinse baths made the meta}! 1s sublected' Thus the manufacture of up of aqueous solutions of the acid corresponding to the P fubmg at SPFedS 11P to 400 feet Per mmute acid radical of the ferric salt of the activating bath, were bmatlPn of lard 011 P taufllw f found partlculafly used. Three different sulfiding baths, prepared as indieffecflye en used In con unct on w1th a sulfide coating cated in the previously examples were also used. These deposlted m accordapce Wlth f f P P- baths are listed below and are referred to by number in Althoughfhe coating of thls lnvfmtlon 1S Pnmanly the tabulation which summarizes the results of these addislgnFd to 31% What may be consldered tempofary P tional examples. tection to ferrous alloy surfaces before and durmg forming, this type of coating may also find use as a good base Activating baths: for paint or other types of corrosion-inhibiting coatings. 1. 300 grams FeCl .6H O in one liter of water, pH It has also been observed that the sulfide coating of this of 1.0 invention will retain sufficient oil or grease to provide temporary protection of ferrous alloy surfaces from.rusting.

We claim:

1. Process for depositing a sulfide coating on the surface of ferrous alloys, comprising the steps of activating said surface ,in an aqueous solution of a ferric salt of a strong mineral acid, rinsing the activated surface in an acid bath having a pH ranging from about 2 to 3, and immersing said activated surface in a sulfiding bath containing solubilized polysulfides.

2. Process in accordance with claim 1 wherein said ferric salt is ferric sulfate.

'3. Process for depositing a sulfide coating on the surface of ferrous alloys, comprising the steps of activating said surface in an aqueous solution of a ferric salt of a strong mineral acid having a pH ranging from 0.3 to 2.0, rinsing the activated surface in an acid bath having a pH ranging from about '2 to about 3, and immersing said activated surface in a sulfiding bath containing solubilized polysulfides and having a pHabove about 7.5.

4. Process in accordance with claim 3 characterized by the additional step of maintaining the pH of said aqueous solution of said ferric salt by adding additional quantities of said ferric salt.

5. Process for depositing a sulfide coating on the surface of ferrous alloys comprising the steps of activating said surface in an aqueous solution of a ferric salt of a strong mineral acid to deposit active ferrous ions thereon, removing any excess of said aqueous solution, rinsing said surface in an acid rinse having a pH ranging from about 2 to about 3, removing any excess of said acid rinse, immersing said surface in a sulfiding bath containing solubilized polys'ulfides to form ferrous sulfide on said surface, rinsing in water and drying said surface.

6. Processfor depositing a sulfide coating on the surface of ferrous alloys, comprising the steps of activating said surface in an aqueous solution, having a pH ranging from 0.3 to 2, of a ferric salt of a strong mineral acid to deposit active ferrous ions thereon, removing any excess of said aqueous solution, rinsing said surface in an acid rinse having a pH ranging from about 2 to 3, removing any .excess of said acid rinse, immersing said surface in a sulfiding bath containing solubilized polysulfides, and having a pH above about 7.5 and rinsing in water and drying 7. Process in accordance with claim 6 wherein said ferricsalt is ferric sulfate.

8. Process in accordance with claim 6 further characterized by forming said sulfiding bath by reacting an inorganic compound of a metal selected from the group consisting of alkali and alkaline earth metals with flowers of sulfur to form said solubilized polysulfides.

9. Process .in accordance with claim 8 wherein said solubilizedpolysulfides are CaS 10. Process for depositing a sulfide coating on the surface of ferrous alloys, comprising the steps of activating said surface in an aqueous solution of a ferric salt of strong mineral acid to deposit active ferrous ions thereon, said aqueous solution having a pH ranging from 0.3 to 2 and being maintained at a temperature above about 160 F.; rinsing said surface in an acid rinse having a pH ranging from about 2 to about 3, and being maintained at a temperature below about F.; removing any excess of said rinse; immersing said surface in a sulfiding bath containing solubilized polysulfides and maintained at a temperature ranging from room temperature to about F.

11. Process in accordance with claim 10 wherein said ferric salt is ferric sulfate, and said acid rinse is an aqueous solution of sulfuric acid.

12. Process for depositing a sulfide coating on the surface of ferrous alloys, comprising the steps of activating said surface in an aqueous solution of a ferric salt of a strong mineral acid to deposit active ferrous ions thereon, said aqueous solution containing a concentration of ferric ions equivalent to from about 2 to grams per liter, and a concentration of ferrous ions equivalent to from about 1 to 78 grams per liter; rinsing said surface in an acid bath having a pH ranging from about 2 to about 3; and immersing said surface in a sulfiding bath containing solubilized polysulfides to form ferric sulfide thereon.

13. Process in accordance with claim 12 wherein said ferric salt is ferric sulfate and said acid bath is an aqueous solution of sulfuric acid.

14. Process in accordance with claim 12 wherein said ferric salt is ferric chloride and said acid bath is an aqueous solution of hydrochloric acid.

15. Process in accordance with claim 12 wherein said ferric salt is ferric nitrate and said acid bath is an aqueous solution of nitric acid.

References Cited in the file of this patent UNITED STATES PATENTS 

1. PROCESS FOR DEPOSITING A SULFIDE COATING ON THE SURFACE OF FERROUS ALLOYS, COMPRISING THE STEPS OF ACTIVATING SAID SURFACE IN AN AQUEOUS SOLUTION OF A FERRIC SALT OF A STRONG MINERAL ACID, RINSING THE ACTIVATED SURFACE IN AN ACID BATH HAVING A PH RANGING FROM ABOUT 2 TO 3, AND IMMERSING SAID ACTIVATED SURFACE IN A SULFIDING BATH CONTAINING SOLUBILIZED POLYSULFIDES. 